1. Field of the Invention
The present invention relates to high frequency filters and filter devices using the filters. The invention also relates to electronic apparatuses incorporating the same.
2. Description of the Related Art
Conventionally, in a high frequency filter, a ground electrode is arranged on one of the main surfaces of a dielectric substrate and microstrip lines are formed on the other main surface of the substrate to form a plurality of resonators. In order to ground parts of the microstrip lines on the other main surface of the dielectric substrate, a through-hole is often used to connect the microstrip lines to the ground electrode.
There is also known another type of high frequency filter using a through-hole as a resonator or a part of the resonator. For example, Japanese Unexamined Patent Application Publication No. 7-86802 describes a high frequency filter using through-holes as resonators. This high frequency filter includes resonators formed by using the inductance and the capacitance of the through-holes. The plurality of resonators are electrically coupled to each other via a capacitance at the gap between electrodes formed on one of the main surfaces of a dielectric substrate to constitute the high frequency filter.
On the other hand, the high frequency filter using the through-holes as resonators has a problem in that it is difficult to adjust the characteristics of the resonators. In other words, when adjusting the characteristics, the diameters of the through-holes need to be changed. However, in order to do so, for example, the dielectric substrate must be replaced, which takes time and increases cost. In addition, since it is difficult to make fine adjustments of the diameters of the through-holes, accurate adjustments are unlikely to be expected.
Furthermore, since the inter-electrode gap, which serves as an additional capacitance element, is used to couple the resonators, the size of the filter increases.
In addition, it is impossible to make the coupling coefficient large by using electrical coupling obtained via the capacitance of the inter-electrode gap. As a result, the high frequency filter cannot obtain a broadband characteristic.
The present invention is able to provide a high frequency filter which facilitates filter-characteristic adjustments and achieves miniaturization. Furthermore, by increasing the inter-resonator coupling coefficient, a broadband characteristic is obtainable. The invention is further able to provide a filter device using the high frequency filter and an electronic apparatus incorporating the same.
To this end, according to a first aspect of the invention, there is provided a high frequency filter including a dielectric substrate, a ground electrode arranged on a main surface of the dielectric substrate, a through-hole, and a plurality of microstrip line resonators formed on the other main surface of the dielectric substrate, a first end of each resonator being grounded via the through-hole. In this filter, the microstrip line resonators share the through-hole for grounding the first end of each resonator, and the resonators are mutually coupled via the inductance of the through-hole.
According to a second aspect of the invention, there is provided a high frequency filter including a dielectric substrate, a ground electrode arranged on a main surface of the dielectric substrate, a through-hole, and two microstrip line resonators formed on the other main surface of the dielectric substrate, a first end of each resonator being grounded via the through-hole. In this filter, the two microstrip line resonators share the through-hole for grounding the first end of each resonator, and the resonators are mutually coupled via the inductance of the through-hole.
In addition, in this filter, the two microstrip line resonators may be spirally formed by being wound in mutually opposite directions.
In addition, a side edge of one of the two microstrip line resonators may be arranged near a side edge of the other microstrip line resonator to mutually couple the resonators inductively.
In addition, in this filter, a second end of one of the two microstrip line resonators may be opposed to the side edge of the other microstrip line resonator to mutually couple the resonators capacitively.
The high frequency filter according to one of the first and second aspects may further include input/output electrodes or wires, each being connected to a point disposed between the first end and the second end of a respective microstrip line resonator.
According to a third aspect of the invention, there is provided a filter device including the high frequency filter according to one of the first and second aspects of the invention.
According to a fourth aspect of the invention, there is provided an electronic apparatus including the high frequency filter according to one of the first and second aspects or the above filter device.
With the arrangements described above, in the high frequency filter and the filter device of the present invention, the filter characteristics can be easily adjusted and miniaturization can be achieved. Moreover, by making the coupling coefficient for coupling between the resonators larger, a broadband characteristic can be obtained.
Furthermore, in the electronic apparatus of the invention, miniaturization, cost reduction, and the improvement of performance capabilities can be achieved.
Other features and advantages of the invention will be understood from the following description of embodiments thereof, with reference to the drawings, in which like references indicate like elements and parts.